9. GaN Nanowall Network: Laser Assisted Molecular Beam Epitaxy Growth and Properties

Low dimensional structures such as two-dimensional (2D) nanowalls, 1D nanorods or nanowires and 0-D quantum dots of semiconductors exhibit different mechanical, electrical and optical properties compared to their bulk counterpart. Despite the promising properties of 1D and 0-D GaN nanostructures, they require complicated and expensive process to handle them individually for fabrication of specific devices. Here, 2D nanowall network draws a special attention due to their continuity in lateral direction and porous surfaces for fruitful applications in the field of nitride based sensors and other nano-scale devices. Among various semiconducting materials, a great attention has been given to the wide, direct band gap III-nitride semiconductors because of their applications in high efficient full color-spectrum light emitting diodes (LEDs), high power electronics devices and ultra-violet photo-detectors among others. This chapter describes the growth of 2D GaN nanowall network on GaN template and sapphire (0001) substrates using laser assisted molecular beam epitaxy (LMBE) technique. The GaN nanowalls of different dimensions were grown by laser ablation of a high purity polycrystalline GaN target in the presence of active r.f. nitrogen plasma. The wall width and pore size were controlled by tuning the laser frequency in the range 10–40 Hz. The structural, optical and electronic properties of the GaN nanowalls were investigated using various characterization techniques such as high resolution X-ray diffraction, field emission scanning electron microscopy, Raman spectroscopy, Rutherford backscattering spectroscopy, photoluminescence and X-ray photoemission spectroscopy. Surface morphology studies exhibited a GaN nanowall network formation of wall width in the range 10–30 nm and pore sizes of 90–180 nm. The photoluminescence spectroscopy measurements showed the optical emission related to GaN nanowalls with a blue shift of about 100 meV from the bulk GaN emission for thinner GaN nanowalls of width <15 nm grown on both GaN template and sapphire (0001) substrates. The enhanced optical band gap of GaN nanowall network is the result of carrier confinement effect of two dimensional electrons when the wall width falls in range of Bohr exciton radius. The KOH wet-etching studies of homo-epitaxial GaN nanowalls confirmed the light emission from 2D GaN nanowall network structure due to quantum confinement.